1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device in which a nonlinear resistance element serving as a switching element is incorporated in each pixel.
2. Description of the Related Art
Recent years, liquid crystal display devices are used not only in relatively simple apparatuses such as watches, clocks and calculators, but also in mass storage information display apparatuses of, for example, personal computers, word processors, terminals of office automation systems, image displays of television sets, etc. In general, a multiplex driving method of matrix display, i.e., a simple matrix method is employed in a liquid crystal display device. In this method, however, due to the inherent characteristics of liquid crystal, the contrast ratio of a display section to a non-display section is of an insufficient value if the number of the scanning electrodes is about 200. It is further degraded if the number of the scanning electrodes is 500 or more. Hence, this method is not suitable for a large scale matrix display.
Under the circumstances, various methods have been developed to overcome the drawbacks of the above-described liquid display devices. One of the methods is an active matrix driving method, i.e., a method of directly driving each pixel by means of a switching element. In this method, a thin-film transistor or a nonlinear resistance elements having nonlinear current/voltage characteristics is used as a switching element.
Conventionally, various semiconductors such as mono-crystalline silicon, cadmium selenide, and tellurium have been proposed as a material of a thin-film transistor. At present, thin-film transistors formed of amorphous silicon are most generally researched. However, to form a liquid crystal display device using a thin-film transistor, a number of fine processing steps are required, resulting in a complicated manufacturing process and a low manufacturing yield. For this reason, the manufacturing cost is inevitably high. Moreover, it is very difficult to produce large scale liquid crystal display apparatuses.
In contrast, a nonlinear resistance element basically has two terminals. It is more simple and can be manufactured more easily as compared to a thin-film transistor which has three terminals. Therefore, by use of the element, the yield of manufacturing the devices can be increased and the manufacturing cost can be reduced.
There are various types of nonlinear resistance element, such as a diode type element formed by joining components made of the same material as a thin-film transistor, a varistor type element using zinc oxide, a metal--insulating layer--metal (MIM) type element, in which an insulating material is interposed between electrodes, and an MSM type element in which a semiconductor layer is interposed between metal electrodes. The MIM element is one of the most simple elements and has been practically used.
In a liquid crystal display device using an MIM element, when a driving voltage is applied across the electrodes between which a liquid crystal layer is interposed, the electrodes are charged at a small time constant. When the driving voltage is not applied, the electrodes are discharged at a large time constant. Thus, the liquid crystal is charged in a short selection period of time after the driving voltage is turned on, and maintains a sufficient voltage for a substantial period of time even after the driving voltage is turned off. Since the effective value of the driving voltage is determined by the voltage applied in the selection period, the ratio of the effective value in the driving voltage ON period to that in the driving voltage Off period can be larger than in a liquid crystal display device employing a multiplex driving method of matrix display. As a result, if the MIM element is used as a switching element, the degradation of the contrast ratio due to the increase in storage of the display device can be greatly reduced as compared to the case of the simple matrix driving method.
However, even if the MIM element is used, the contrast ratio may be degraded as in the simple matrix driving method, in the matrix display on large scale with 500 or more scanning lines. To overcome this drawback, there is provided a structure wherein each wiring electrode is divided into two at its central portion, thereby dividing the entire display screen into two blocks, each of which is driven independently, thus halving the apparent number of scanning lines.
In general, in a liquid crystal display device having an MIM element, since the insulating layer of the MIM element has a relatively small thickness of 500 to 700 .ANG., the withstand voltage of the layer is low. Hence, dielectric breakdown easily occurs in the MIM element owing to the static electricity generated during the process of manufacturing the device. For example, the process of manufacturing a liquid crystal display comprises a step of rubbing the substrate with cloth after an orientation film is formed on the substrate. Static electricity of 500 V to 20 kV tends to be generated especially in the rubbing step. It is difficult to prevent the generation of static electricity. In general, since electric charge concentrates at the end portions of a wiring electrode, if the display screen is divided into a plurality of blocks as described above, electric charge concentrates at the dividing portion of each wiring electrode. For this reason, a difference between the potentials of the division ends of each wiring electrode arises, and the electricity is discharged from the division ends to the display electrodes adjacent to the division ends. This discharge causes puncture of the MIM elements. The puncture or dielectric breakdown results in characteristic defects of the element, which are represented as display defects in pixel units, i.e., point defects. The point defects concentrate at the dividing portion, i.e., the boundary between the blocks of a display screen.
The applicant inspected pixel defects appearing in the boundary between the blocks of a display, and found that about half of all the pixels adjacent to the boundary were defective.
To prevent the occurrence of point defects due to static electricity, several methods have been proposed. For example, Published Unexamined Japanese Patent Application (PUJPA) No. 62-58226 discloses a structure wherein dummy pixels are provided outside the display screen. Static electricity is discharged through MIM elements which are connected to the dummy pixels, that is, the MIM elements are punctured by the static electricity, thereby preventing the breakdown of the MIM element arranged in the display section. However, this structure cannot be applied to a liquid crystal display device wherein the display section is divided into a plurality of blocks. Therefore, another method for preventing the breakdown of the element caused by static electricity is greatly in demand.